OF THE MICROSCOPE.. 141 



frog rolled up in it and laid on the plate, where he is held by a 

 few turns of light packing twine passed into the slits in the side 

 of the plate and carried from one to the other and over the 

 animal. Small frogs are best for this purpose, but when too 

 small they are not easily handled. The position of the animal 

 on the plute is so arranged that the foot may be spread over the 

 glass plate that fills the large opening. 



The plate is attached to the stage as follows: A cylindrical 

 brass block (Fig. 40) is provided this block having a milled belt, 

 which renders it more easily turned. The upper surface of 

 this block receives a screw which passes 

 through a slot of considerable length, cut in 

 the frog plate, thus allowing a wide range of 

 motion on the part of the latter; the under 

 surface of the block receives a second screw, 

 which serves to secure it to the stage of the 

 microscope, as shown in Fig. 39. The holes 

 for these screws are not in the same line, 

 their axes being about a quarter of an inch 

 Fig. 40. ' apart, and the consequence is that when the 



brass block is rotated on the stage, the screw 

 that passes through the plate acts like a crank in relation to 

 the plate, and moves it longitudinally, provided it (the plate) is 

 kept from rotating with the block. The upper screw is inserted 

 with sufficient tightness to keep the plate from shaking, but is 

 left so loose that the plate can be readily moved back and forth. 

 Hence, while the plate is attached to the stage in such a way 

 that it can not tip up or fall off, it may readily be mo\ed in two 

 directions, one the arc of a comparatively large circle, and the 

 other a longitudinal motion at right angles to this. 



This frog plate forms in fact a sort of mechanical stage which 

 admits of very delicate movements being very steadily made. 

 Where this plate is used, the microscope may be inclined to 

 any angle, and no jerking or starting of the animal can displace 

 the portion of the foot that is under observation. Difi'erent 

 parts of the same foot and different corresponding parts 01 dif- 

 ferent feet are more or less suited to purposes of observation, 

 according as they are more or less transparent and more 

 or less fully supplied with vessels. It is therefore of great ad- 



